A rare case of the Zika virus being transmitted through sex, not a mosquito bite, has been reported in the US.

A patient infected in Dallas, Texas, is likely to have been infected by sexual contact, the Centers for Disease Control (CDC) told the BBC.

The person had not travelled to infected areas but their partner had returned from Venezuela.

Zika is carried by Aedes mosquitoes and has been linked to thousands of babies being born with underdeveloped brains.

It is spreading through the Americas and the World Health Organization has declared the virus a global public health emergency.

In a related development, two cases of the Zika virus have been confirmed in Australia. Officials
said the two Sydney residents had recently returned from the Caribbean.

Meanwhile, Brazil - the country worst hit by the outbreak - has revealed it is investigating 3,670 suspected cases of microcephaly in babies linked to the Zika virus.A total of 404 cases
have so far been confirmed - up from 270 last week - while 709 cases have been discarded, the country's health ministry said.

However, this is not a new HIV/Aids moment. HIV infection is incurable and dramatically shortens lives without daily medication. Zika infections are short, mild and pose a significant threat only in pregnancy.

The ministry also said 76 infant deaths from microcephaly, either during pregnancy or just after birth, were suspected. The case in Dallas would be the first known infection to take place in the mainland US, though Texas has seen seven other Zika cases all related to foreign travel.

Anne Schuchat, principal deputy director for CDC, said this was the first case it had dealt with involving a "non-traveller". "We don't believe this was spread through mosquito bites, but we do believe it was spread
through a sexual contact." A statement issued by the CDC said the best way to avoid Zika virus infection was "to prevent mosquito bites AND to avoid exposure to semen from someone who has been exposed to Zika".

The CDC recommends that pregnant women avoid travel to more than two dozen countries with Zika outbreaks, mostly in the Caribbean and Latin America, including Venezuela.
Brazilian President Dilma Rousseff said her government was focused on eradicating the mosquito that transmits the virus.